unity – Gravity is working in reverse

I am new to Game Development. In fact, this is my first project. I am trying to make a speed run/parkour game. I decided to start with movement and physics, and make the course later. Movement works fine, although gravity works in a unexpected way. Instead of holding the player down, it makes the player float up. I know JavaScript, CSS, HTML, and Python but not C#. So I got the code from a video. Here’s the link to the video: https://bit.ly/317EUW1. Here’s the code:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class PlayerMovement : MonoBehaviour
    public CharacterController controller;

    public float speed = 12f;
    public float gravity = -9.81f;

    public Transform groundCheck;
    public float groundDistance = 0.4f;
    public LayerMask groundMask;

    Vector3 velocity;
    bool isGrounded;

    // Update is called once per frame
    void Update()
        isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);

        if(isGrounded && velocity.y < 0)
            velocity.y = -2f;

        float x = Input.GetAxis("Horizontal");
        float z = Input.GetAxis("Vertical");

        Vector3 move = transform.right * x + transform.forward * z;

        controller.Move(move * speed * Time.deltaTime);

        velocity.y += gravity * Time.deltaTime;

        controller.Move(velocity * Time.deltaTime);


Please get back to me as soon as possible.